1. Field of the Invention
The present invention relates to a semiconductor device which uses a MISFET (Metal Insulator Semiconductor Field Effect Transistor) as a diode.
2. Description of the Background Art
FIG. 37 shows a schematic symbol which represents the diode. As is well known, the diode permits current flow in the forward direction (FWD direction) from the anode (A) to the cathode (K) but inhibits current flow in the reverse direction (REV direction) from the cathode (K) to the anode (A).
While specific methods for forming diodes include methods utilizing the PN junction or the Schottky barrier, a diode using a MISFET is described herein.
FIG. 38 is a diagram that illustrates a diode using a MISFET. As shown in FIG. 38, the gate G and the drain D of the MISFET are short-circuited together so that the gate G and the drain D function as the anode and the source S functions as the cathode. In FIG. 38, the body B of the MISFET is grounded.
In the case of the diode using a MISFET, its current-voltage characteristic varies depending on the value of the threshold Vth for formation of the channel in the MISFET. FIG. 39 is a graph that shows two current-voltage characteristics G2 and G3 with different threshold values Vth. The characteristic G2 has Vth=0 [V] and the characteristic G3 has Vth=2 [V]. The vertical axis in FIG. 39 shows the drain-source current ID and the horizontal axis shows the drain-source voltage VD (in the diode configuration, the voltage VD is also the gate-source voltage VG since the drain and the gate are short-circuited).
Now, in the diode, it is ideal that the current infinitely flow when it is forward (FWD) biased and the current is zero when reverse (REV) biased. Accordingly, in the graph of FIG. 39, it is desired that the current-voltage characteristic steeply rise when forward-biased, i.e. in the FWD region where VD≧0.
That is, this means that the characteristic G2 with threshold Vth=0 [V] is preferable to the characteristic G3 with threshold Vth=2 [V]. With the forward bias, while the characteristic G3 involves a voltage loss because it needs application of an extra amount of gate-source voltage VG for the threshold Vth=2 [V], the characteristic G2 does not involve such a voltage loss.
In the MISFET, the threshold Vth can be varied in accordance with parameters such as the impurity concentration in the channel region, the dielectric constant of the substrate material, etc. It is therefore easy to realize the characteristic G2 by adjusting such parameters.
However, in the case of the characteristic G2, a leakage current IL flows even when the voltage VD is zero. Furthermore, even with the reverse (REV direction) bias, the influence of the leakage current IL remains when the voltage VD is near zero.
For use as a diode, the leakage current occurring when the reverse bias is applied must be suppressed below a given standard value. Therefore the characteristic G2 cannot be adopted and the threshold Vth must be set at larger value as that of the characteristic G3.